In a typical timing transmission in a vehicle engine, an endless, flexible, traveling chain transmits rotation from a crankshaft sprocket to one or more camshaft sprockets. A ratchet tensioner is typically used to maintain proper tension in the slack side of the timing chain, that is, the side moving from away the crankshaft sprocket, in order to suppress vibration. The ratchet tensioner has a protruding plunger, and is mounted on the engine body in such a way that a front end of its plunger presses against the back of a pivoted tensioner lever, urging a shoe on the tensioner lever into sliding contact with the chain. A typical timing transmission using such a tensioner is described in U.S. Pat. No. 6,478,703.
In the case of a balancer, two balance shafts are provided with gears that mesh with each other, and a balancer chain is in mesh with a sprocket mounted on one balance shaft and a crankshaft sprocket. The balancer also includes a ratchet tensioner for maintaining proper tension in the balancer chain, as described in Japanese Laid-open Patent Publication No. 2004-308624.
An example of a conventional ratchet type tensioner is shown in FIG. 7. In the tensioner 21, a plunger 24 is slidable in a plunger-accommodating hole 23 in a tensioner body 22, and protrudes forward from the front surface of the tensioner body. A compression spring 26 bears against the bottom of the plunger-accommodating hole in the tensioner body and extends through an opening at the rear of the plunger into a blind hole 25 inside the plunger 24, its opposite end bearing against the closed end of the blind hole. The spring 26 continuously biases the plunger 24 in the protruding direction.
A longitudinal rack 27 is formed on the outer circumferential surface of the plunger 24. The rack 27 comprises a series of rack teeth disposed at a uniform pitch over the entire length of the rack. The length of the rack is determined in accordance with the expected amount of loosening of the timing chain.
A pawl 29 is pivotably supported on the tensioner body 22 by a shaft 28 at a position adjacent the rack 27. The pawl 29 is continuously biased clockwise by a spring 30 held in compression between the pawl and the bottom of a spring-receiving hole in the tensioner body. A first pawl tooth 29a, and a second pawl tooth 29b spaced rearward of the first pawl tooth 29a, are engageable with the rack teeth 27a. The pawl 29 and the rack teeth 27a form a “backstop” mechanism 31 that blocks backward displacement of the plunger 24.
The first pawl tooth 29a, formed on the front side of the outer edge of the pawl 29, has a triangular shape corresponding to the shape of rack teeth 27a, and engages with a space between rack teeth substantially without a clearance. The second pawl tooth 29b is smaller than the first pawl tooth 29a. That is, its height, measured along a direction approximately perpendicular to the plunger axis, is lower than the height of pawl tooth 29a. Tooth 29b is formed at a location spaced rearward from tooth 29b by at least three times the rack tooth pitch. When the first pawl tooth 29a is engaged between two rack teeth 27a substantially without any clearance, the second pawl tooth 29b is out of contact with the rack teeth 27a, and does not restrict movement of the plunger 24. When the chain loosens by an amount permitting the plunger to protrude through a distance corresponding to half the width of one rack tooth 27a, the plunger 24 begins to move forward and the first pawl tooth 29a rides over the top of a rack tooth 27a. The second pawl tooth 29b then comes into contact with another rack tooth 27a, causing the pawl to rotate clockwise, and causing the first pawl tooth 29a to be engaged with another rack tooth 27a. While the second pawl tooth 29a is between two rack teeth, the plunger 24 can move slightly forward or backward in accordance with the looseness or tension in the chain, and the plunger 24 applies proper tension to the chain.
Both in the case of a timing chain and in the case of a chain for driving auxiliary equipment such as a balancer or the like, there are engines in which it would be desirable to have the backstop mechanism of a ratchet tensioner function only when the chain has become elongated after a lengthy period of operation. However, in a conventional ratchet tensioner, the length of the rack is such that it is opposite the ratchet pawl throughout the entire range of movement of the plunger. In such a tensioner, if the backstop mechanism is effective to prevent jumping of the chain on the sprocket teeth when the chain becomes elongated, it can exhibit insufficient backlash in the initial stages of its operation before the chain has become elongated. The tensioner cannot operate reliably in an engine that requires a tensioner having a backstop mechanism that functions in one region of plunger movement and does not function in another region of plunger movement.
Accordingly, an object of the invention is to solve the above-mentioned problems and to provide a ratchet tensioner that can be adapted to an engine of the kind that requires a tensioner having a backstop mechanism that functions only when the chain has become elongated, and does not function in the initial stages of operation of the chain.